Familial influences on tests of mental and motor performance at ages four and seven were examined by comparing correlation coefficients among monozygotic twins, dizygotic twins, full siblings, and half siblings, within race, sex, and social class groups. The correlations between scores of twin and sibling pairs on the Stanford-Binet (age 4) and Wechsler (age 7) intelligence tests suggested a greater genetic influence than was found for infant test scores. The monozygotic-dizygotic twin differences and full sibling-half sibling differences in correlations indicated more of a genetic influence for height and weight at ages four and seven than for the cognitive measures.